RWS, it's not an assumption, people are guilty by association. Here's a legal example, you are out at a bar with a freind who violently assaults someone as you're leaving. An hour later the Police find you and you get arrested for complicity (a charge like aiding and abetting, rendering criminal assistence, accessory during the fact, failure to report the crime or render assistance when an injury is involved, etc...), because you refuse to give a statement implicating your friend.

Even though you may not have actually commited the crime, you are still guilty by association.

The same is true if you drive to a convienence store and wait in the car while your companion goes in. Unbeknownst to you, your friend boosts a case of beer (or anything for that matter), gets in the car and you drive off. You get pulled over a few blocks away, and guess what? You both get charged with shoplifting.

Guilt by association. Even being ignorant of the facts is not a legitimate legal defense. So my approach is don't associate. That might be considered anti-social behavior, but I know I won't ever catch a social disease. lol